Titanium and titanium alloys have become important structural metals due to an unusual combination of properties. These alloys have strength comparable to many stainless steels at much lighter weight. Additionally, they display excellent corrosion resistance, superior to that of aluminum and sometimes greater than that of stainless steel. Further, titanium is one of the most abundant metals in the earth's crust and, as production methods become more economical, it will be employed in ever growing applications.
Various alloys of titanium and nickel are part of the alloy class known as shape memory alloys (SMAs). This term is applied to that group of metallic materials (also known as nickel-titanium alloys) that demonstrate the ability to return to a defined shape or size with thermal processing. In a most general sense, these materials can be plastically deformed at some relatively low temperature and return to their pre-deformation shape upon some exposure to higher temperatures. This shape memory effect (as it is sometimes called) i.e., the ability to exhibit a temperature dependent change in shape or configures, finds numerous commercial, especially medical, applications.
Nickel-titanium SMAs undergo a phase transformation in their crystalline structure when cooled through a transition temperature from the relatively stronger, high temperature or “Austenite (or austenitic)” form to the relatively weaker, low temperature or “Martensite (or martensitic)” form. Such crystalline transformations are responsible for the hallmark characteristics of these materials; their thermal, or shape memory; and their mechanical memory.
The characteristics of titanium and titanium-based alloys (conversely nickel-titanium alloys), especially their shape memory, means they have been widely used as components of medical devices such as catheters, stents, guidewires, blood filters, stylettes, and numerous other devices.
A major limitation in the use of titanium and nickel-titanium alloys has been the difficulty of joining these materials to other materials. Because of its high cost, it is often desirable to limit the use of nickel-titanium to the actual moving parts of a device, while fabricating supporting members from less expensive materials such as stainless steel or other ferrous metals. However, welding of nickel-titanium to stainless steel and to ferrous metals in general has proved to be particularly difficult, as disclosed by Ge Wang, in a review “Welding of Nitinol to Stainless Steel.”
In addition, the reactivity of titanium makes it important that any welding be done in a clean, inert atmosphere e.g., as argon blanket, or in a vacuum, to reduce the tendency to form damaging oxides or nitrides. Nickel-titanium alloys materials naturally form surface oxides in air during processing into finished form making the use of an inert atmosphere (or vacuum) of lesser importance. The principal surface oxide formed is TiO2.
However, the difficulty of joining nickel-titanium to other materials, such as stainless steel, has remained exceedingly limiting to the art. Many techniques have been employed with limited success. Non-fusion joining methods are most commonly used to join nickel-titanium; including soldering, epoxies and other adhesives; and various types of mechanical joining such as crimping. These techniques are not without their drawbacks. Soldering, for example, must often be accomplished with special flux to remove and inhibit the formation of surface oxides during soldering. Epoxies and adhesives are not suitable for all manufacturing techniques and types of uses to which these nickel-titanium products are directed. Mechanical fastening may cause over deformation and cracking of the nickel-titanium. Interference fit or the interlocking of components has been successful, but requires manufacturing to close dimensional tolerances.
Various methods have been used to attempt to improve results in welding of titanium alloys to ferrous metals. Those methods are variously described in the following United States patents which are incorporated by reference in their entireties herein:
U.S. Pat. No. 4,674,675
U.S. Pat. No. 3,038,988
U.S. Pat. No. 4,708,282
U.S. Pat. No. 6,410,165
U.S. Pat. No. 6,875,949 to Peter C. Hall, also incorporated by reference herein discloses a method of welding titanium metals and ferrous metals using nickel or iron added to the weld pool. The '949 patent discourages the use of aluminum, chromium and titanium (col. 8, line 49, et seq.) stating that they do not improve weld quality between titanium metals and ferrous metals.
Accordingly, the art has needed a means for improving the art of fusion welding titanium, and titanium-based alloys, to ferrous metals. In its most general sense, the present invention overcomes the problems experienced in this art and provides an improved method of welding titanium, or titanium-based alloys, to ferrous metals e.g., metals, steel, other alloys, eutectic mixtures containing or comprising any appreciable amount of iron. A preferred embodiment of this invention is an improved method of welding nickel-titanium alloy e.g., the alloy known as nitinol, and stainless steel. This invention is particularly applicable to the medical device art, e.g., guidewire.